This invention relates to power operated fastening devices, and in particular, to an apparatus and method for installing fasteners.
A blind rivet is generally composed of two pieces. The first piece is the mandrel or stem, a cylindrical member having an enlarged head at one end. The second piece is the sleeve, a generally tubular member abutting the stem head, with an outwardly protruding flange at its end opposite the stem head. In use, such a fastener is typically placed in the pulling head of a powered fastener installation device and directed to the workpiece. In other instances, the fastener is placed in the workpiece and the powered fastener installation device then applied to it. The stem of the fastener is gripped by the jaws of the fastener installation device pulling head, which is then operated by hydraulic pressure to clamp the jaws radially about the fastener stem and pull the stem rearward away from the workpiece. The tension on the stem pulls the stem head against the sleeve, thereby deforming the sleeve against the rear surface of the workpiece. The fastener is constructed so that the stem breaks off at a predetermined load limit and at a predetermined location on the stem. A fastener may also be set by a powered fastener installation device by rotating the stem, thereby breaking off the stem on reaching a pre-determined rotational stress limit.
Traditionally, installation of such fasteners has been performed by an operator using a hand-held powered fastener installation apparatus. More recently, however, the powered fastener installation has been accomplished in conjunction with industrial robots or other automated positioning devices. Use of such robots has many attendant benefits in the manufacturing process. However, their use also has disadvantages in the installation of fasteners. For example, powered fastener installation devices used in conjunction with industrial robots have traditionally operated quite slowly. A human operator experienced in powered fastener installation can install fasteners with a comparable or superior rate of speed for at least a short period of time. Another drawback is that robot mounted powered fastener installation devices risk damaging the workpiece. For instance, if the jaws of the pulling head become fouled, the jaws may slip along the fastener stem, resulting in a loss of pulling force and a partially upset fastener. If the robot withdraws the powered fastener installation apparatus from the workpiece with the jaws clamped to a partially upset fastener, the fastener may be pulled from the workpiece aperture, possibly deforming the workpiece surface. Such a mishap can ruin a very expensive workpiece. On the other hand, the jaws could slip entirely off the partially upset fastener and leave the fastener protruding from the workpiece. Although this problem does not damage the workpiece in and of itself, it reduces the efficiency of the device, fails to fasten the workpiece, possibly interferes with the robot's movement, and leaves a fastener projecting from the workpiece, creating a potentially damaging or hazardous lever--which can deform the workpiece--or projectile--which can injure the operator or damage the workpiece or machinery.
Another drawback with robot-mounted powered fastener installation devices of the prior art is that they install the fasteners by a "blind" pull. That is, the pulling head pulls on the fastener stem until a pre-determined time has elapsed, a pre-determined pressure has been reached, or until it has pulled for a pre-determined length. None of these methods directly indicate whether the fastener stem has been broken. Thus, the robot can be fooled into thinking that the fastener has been upset. Slipping of the jaws on the fastener stem or some other problem can lead to failure to install the fastener, or worse yet, withdrawal of a partially upset fastener through the workpiece aperture.
A similar problem can occur in the prior art devices if the robot or workpiece is slightly misaligned so that the fastener is not inserted into the workpiece aperture, but instead abuts the workpiece surface. The fastener may be upset into thin air, fooling the robot into thinking that it has been successfully installed. An associated problem is that deflection of the fastening device due to slight misalignment is transferred either to the workpiece or to the robot, risking damage or misalignment of both.
A further drawback to the robot-mounted powered fastener installation devices of the prior art is their susceptibility to fastener feed problems resulting from the irregular shape of the fasteners and the complexity of the mechanism. Robot mounted fastener installation devices are notorious for their jamming problems. To further compound this problem, pneumatic conveying of fasteners can result in partial or total disassembly of the fastener. As a result, such devices are reputed to be unreliable and costly in terms of downtime.
One characteristic of robot-mounted powered fastener devices that is incompatible with many standard powered fastening devices is that the robot arm may be called upon to insert the fastener in any orientation, including facing down. Thus, unless the fastener is positively held in place, it may fall out by the force of gravity before it is installed in the workpiece. The robot would have no way of knowing that the fastener had fallen out. Consequently, the prior art has endeavored to mechanically grip the fastener throughout the cycle. Mechanical gripping, however, makes it difficult to purge the apparatus if required.